To produce a solid electrolyte fuel cell stack structure by stacking separators and electrochemical cells alternately, it is necessary to supply a gas between the separator and the single cell so that fuel or oxidizing agent is supplied to electrodes of the single cell. At the same time, the single cell and the separator must be electrically connected in series by inserting a conductive connecting member (interconnector) between the separator and the single cell. When the above-described conductive connecting member is located in, for example, a fuel gas passage, it must be chemically stable in a reductive fuel gas at an operating temperature of the single cell. Further, there is required a gap through which the fuel gas can pass. For such reasons, so-called nickel felt has been commonly used as the connecting member in the fuel gas passage.
However, gas permeability is reduced in a state that the nickel felt is pressured, so that power generation efficiency tends to be lowered. The applicant has therefore disclosed that a metal mesh is embossed to form a protruding tongue piece and this tongue piece is pressed to the electrodes of the electrochemical cell to thereby realize the electrical conduction in Japanese Patent Laid-open Publication No. 2007-265896A. By doing so, it is possible to assure the gas permeability and make a press load onto an electrochemical cell uniform.
In the above-described stack structure, it is desired that an air electrode and fuel electrode of the cell and the conductive connecting member are bonded, whereby strength of the bonding portion is improved and the electrical conduction is stabilized. However, the conductive connecting member is normally made of a heat resistant metal having a high melting point, and the electrode is made of ceramics having relatively high electrical conductivity. In general, these heterogeneous materials are hard to be bonded together at a high mechanical strength at 1000° C. or lower.
For example, Japanese Patent Laid-open Publication No. 2005-339904A discloses that bonding between both of the interconnector and the cell is tried using conductive ceramics for the purpose of firmly bonding the interconnector and the cell. Specifically, La—Sr—Co—Fe-based perovskite-type complex oxide is used.
In addition, Japanese Patent Laid-open Publication No. 2005-050636A discloses that contact materials for the air electrode are formed using a mixture of silver powder/silver alloy and the perovskite-type complex oxide powder.
Further, Japanese Patent Laid-open Publication No. 2009-016351A discloses that a cathode composed of a manganese spinel compound is bonded to a metal interconnector using an adhesive agent composed of a manganese spinel compound as well (0029).
However, in the additive disclosed in Japanese Patent Laid-open Publication No. 2005-339904A, a heat treatment at a high temperature of 1000° C. or higher is required for the purpose of realizing sufficiently high strength. At the above-described high temperature, the metal interconnector is extremely oxidized, and therefore, the contact resistance is increased by the generation of chromia (Cr2O3). On the other hand, for the purpose of suppressing and preventing oxidation of the metal interconnector, when the heat treatment is performed, for example, at a temperature of 800 to 900° C., the bonding agent is not sufficiently sintered. Therefore, there is the possibility that the desired bonding strength is not obtained and a joining may be broken down during operation of the cell. In addition, the contact resistance is large due to the insufficient sintering of the bonding agent.
In the bonding agent disclosed in Japanese Patent Laid-open Publication No. 2005-050636A, the sintering property at a low temperature is improved and an increase in the resistance caused by oxidation of the metal interconnector can be suppressed. However, there is a problem that the cost is high, since silver is expensive.
Japanese Patent Laid-open Publication No. 2009-016351A discloses that a cathode composed of a manganese spinel compound is bonded to the metal interconnector using an adhesive agent composed of a manganese spinel compound as well; but there is not a specific example. In general, the manganese spinel compound is manufactured by mixing manganese oxide powder and cobalt oxide powder in a spinel ratio to sinter the mixture.
The present inventors have tried that the manganese spinel compound is generated between the cathode and the stainless steel interconnector to bond the cathode and the interconnector using the above-described manufacturing process. However, the inventors have found that the bonding strength between the cathode and the interconnector is low at the joint temperature of less than 1000° C., and moreover, the manufacturing variation in the bonding strength is large in the manufacturing process. When the variation in the bonding strength is increased during manufacturing, reliability in the joint is low and electrical conductivity is lost.